Recently a new driving technique has been developed for motors electronically switched according to a preestablished profile. The profile is typically sinusoidal, and may be digitized and stored in a ROM memory in the form of an N number of samples. A driving current or voltage corresponding to such a predefined profile is forced into each respective phase winding of a motor in synchronism with the rotor's position. The position may be detected by way of dedicated sensors or, as generally done, by sensing the zero crossing instants of back-electromotive-forces induced on the phase windings of the motor.
Usually this synchronizing function is carried out by measuring the interval of time (period) between two successive zero crossings of the back-electromotive-force signals. A frequency multiplier (e.g. a clock signal divider) is used to drive the pointer of the digital sample of the predefined and stored profile, represented by the N digital samples stored in the ROM memory. This synchronizes the phase switchings with this instantaneous value of the period occurring between two successive zero crossings. An entirely digital driving system of this kind is described in the prior European patent application No. 96830295.0. Dedicated reconstruction systems of back-electromotive-force signals to synchronize the driving of sensorless motors are described in the prior European patent applications No. 96830440.2 and 97830353.5, assigned to the assignee of the present invention.
These entirely digital driving systems may be alternatively commanded to operate in a closed loop mode or in an open loop mode of the synchronization control. In the open loop mode the switching frequency may be imposed by writing into a dedicated register certain frequency data for driving the phase switching to correspond to N times the desired rotational speed. When the system is commanded to function again in a closed loop mode, for example, during accelerations or decelerations from a steady state speed, a command is sent to ensure that the synchronizing system uses for the successive phase switching the time period between a forced synchronization signal, that is, of a pseudo zero crossing, and the pulse of the first real zero crossing. The successive scanning periods will use the time interval between two successive real zero crossings according to a normal closed loop mode of synchronization.
The ability to switch from an open loop and a closed loop mode of operation is implemented in the known systems by way of relatively complex circuitry. Moreover, in systems functioning in a voltage mode it is necessary to employ special means for introducing a certain phase shift for compensating for the phase angle existing between the drive current flowing in the windings and the voltage applied thereto.